Tree Encroachment Induces Biotic Differentiation in Sphagnum -Dominated Bogs

Abstract

This study aims to understand the effects of recent tree encroachment on plant richness and diversity of Sphagnum-dominated bogs isolated in an agricultural landscape. A nested paired sampling design was used to compare plant species richness and beta diversity between open and forested habitats of 14 bogs in southern Québec, Canada. We evaluated the impact of tree encroachment at regional and local scales (between and within bogs, respectively). Tree basal area, canopy openness and stand age were evaluated in forested habitats. We used permutation paired sample t-tests to compare species richness between open and forested sites. Beta diversity was calculated as between-site similarities in composition, and differences were evaluated using tests for homogeneity in multivariate dispersion. Forested habitats had greater species richness than open habitats due to enrichment by facultative and non-peatland species as well as by mid- and shade-tolerant vascular plants. At both scales, this species enrichment was associated with flora differentiation (increase of beta diversity), although at regional scale, this was true for bryophytes only. Tree basal area had a positive influence on forested habitats species richness. These compositional changes are expected to increase similarity between bog flora and upland vegetation, and consequently decrease regional diversity.

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